Fuel cells, with the function of transforming chemical energy from fuel and oxidant into electric energy directly, is not involved in the form of heat engine and thus does not restricted by the Camot cycle. It also has high energy efficiency and be widely considered as the most potential energy-conversion devices for the 21st century. Proton Exchange Membrane Fuel Cell (PEMFC) has emerged as the most promising power source for a broad range of applications including automotive power, mobile power, and small power plant. In the past ten years, PEMFC has drawn much attention, but there are still many difficulties in its commercialization procedure. In order to maintain a well proton conductivity of the membrane, the reactant gases are usually humidified during the fuel cell operations since PEMFC uses polymer solid polymer as proton conductor and good proton conduct process will be obtained if an operation of the solid polymer is in hydration status only.
Fuel cell performances are usually characterized by its polarization behavior through I-V curve, both of the steady and the transient state will be considered in its performance. In the application of the fuel cell, non-steady or transient operations are inevitable, such as the startup and shutdown of the cell, temperature change and load change or fluctuation of reaction gas. It arises a new problem that whether these transient responses of fuel cell can satisfy the commercial applications. In the last few years, the transient behavior of the PEMFC has already drawn more and more scientists' attentions. The related documents are: S. Kim, S. Shimpalee, J. W. V Zee, J. Power Sources, 135, 110 (2004); S. Yerramalla, A. Davari, A. feliachi, T. Biswas, J. Power Sources, 124, 104 (2003).
However, in order to simplify the system designation and optimize the operating conditions, researchers have been aiming at operating the fuel cell with unhumidified gases goal. The related articles are F. N. Buchi, S. Srinivasan, J. Electrochem. Soc., 144, 2767 (1997) Z. Qi, A. Kaufman, J. Power Sources, 109, 469 (2002); TH. Yang, Y G Yoon, C. S. Kim, S. H. Kwask, K. H. Yoon, J. Power Sources, 106, 328 (2002); S. H. Kwak, T H. Yang, C. S. Kim, K. H. Yoon, J. Power Sources, 118, 200 (2003); M. V Williams, H. R. Kunz, J. M. Fenton, J. Power Sources, 135, 122 (2004); R. Eckl, W. Zehtner, C. Leu, U. Wagner, J. Power Sources, 138, 137 (2004). But no successful examples with effective methods to avoid membrane dehydration and the fuel cells flooding have been reported among these papers.
In researches on dynamic behavior of the fuel cells, J. Hamelin et. al (J. Hamelin, K. Agbossou, A. Laperriere, F. Laurencelle, T. K. Bose, Int. J. Hydrogen Energy, 26, 625 (2001)) found out the polarization hysteresis of an MK5 fuel cell stack. Load change (positive or negative) was controlled within every 0.15 s and with humidified gases. The results showed that the performance of negative load changes was always better than the positive ones. Then this phenomena was explained as a relationship of the membrane hydration level and the proton conductivity. During the positive load change, generated water needs time to distribute in the fuel cell which results a worse performance, and it is just the opposite effect for the negative load change. When the load decreases, as water in the membrane supports the proton conduct, the corresponding performance (I-V) curve shows better performance, which is called hysteresis.
J. R. Atkins et. al (J. R. Atkins, S. C. Savett, S. E. Creager, J. Power Sources, 128, 201 (2004)) explored the performance response when the membrane dehydrates. They found that the cell current and the high frequency resistance shows periodical fluctuation as the relative humidity of the reactant gases decrease. They explained this by the periodical dehydration and hydration of the membrane. S. Kim et. al (S. Kim, S. Shimpalee, J. W. V. Zee, J. Power Sources, 137, 43 (2004)) also investigated the transient behavior of the PEMFC with the humidified gases by fixed flow rates.
Actually, the electrical interface of the fuel cell is unfixed during the transient operation. K. Kanamura et. al (K. Kanamura, H. Morikawa, T Umegaki, J. Electrochem. Soc., 150, A193 (2003)) found that the hydrophilic/hydrophobic interface between Pt electrode and Nafion membrane in HClO4 solution is very easy to drift as the change of the relative humidity.
Simulation results also show that the reactant interface drifts as the operating condition change. The related articles are: C. Ziegler, H. M. Yu, J. O. Schumacher, 3rd European Polymer Electrolyte Fuel Cell Forum, B064-098, Luceme, Switzerland, (2005); C. Ziegler, H. M. Yu, J. O. Schumacher, J. Electrochem. Soc., 152, A1555 (2005).